结论：开模是一道成本门槛，门槛之前要把结构问题解决掉
塑料件和压铸件需要开模具，模具的成本和周期都不小，而且一旦开出来，结构的改动余地就很有限 ——小的尺寸调整可以通过修模实现，结构性的改动往往意味着重新开模。
所以开模前应当完成的验证，就是那些「如果不合格需要改结构」的项目。
常见的失误是：结构设计完成后直接开模，样件出来才发现强度不足或者装配有问题，这时改动的代价已经很高。
开模前应当完成的验证
强度与刚度的核算与验证。 关键受力部件能否承受设计载荷。
装配关系的验证。 与相邻零件的配合、装配的可行性。
功能的验证。 活动部件的运动、卡扣的扣合、密封的实现。
尺寸链的核算。 累积公差是否会导致装配问题。
材料的确认。 选定材料的实际性能是否满足要求。
工艺可行性的初判。 壁厚、脱模、浇口位置是否合理。
这几项中，强度和装配是最需要实物验证的，计算能提供参考但不能完全替代。
开模前的验证方法
模具还没有，怎么做实物验证？可行的途径：
快速成型件。 用增材制造做样件。注意材料性能与注塑件差别很大，可用于验证装配和尺寸，不宜直接用于强度验证。
机加工件。 用同种材料的板材或棒材加工出零件。材料性能接近实际，可以用于强度验证，但纤维取向和内应力与注塑件不同。
简易模具。 软模或简易钢模，成本低于正式模具，能做出接近实际的样件。
相似件替代。 用已有的相似零件做原理验证。
局部试样。 只做关键受力部位的试样，做针对性的强度验证。
「机加工件做强度验证」是性价比较高的做法。 虽然与注塑件有差异，但能回答「这个结构、这个材料、这个壁厚，够不够强」这个核心问题。
强度验证的重点
关键受力部位。 承载路径上的部件。
应力集中处。 孔、缺口、壁厚突变、加强筋根部。
装配连接处。 螺钉座、卡扣、嵌件周围。
薄弱截面。 为了脱模或减重而减薄的位置。
长悬臂。 脚踏、扶手这类伸出的结构。
验证的方式是加载至失效，观察在哪里失效、失效载荷多少，据此判断余量。
「加载至失效」在这个阶段比「判断是否合格」更有价值，因为它能说明余量的大小和薄弱点的位置。
余量的判断
开模前的验证结果，余量应当留得比定型检验时更多，原因是：
样件与量产件的差异。 工艺不同会影响性能。
材料批次的分散。 量产时材料会有批次差异。
工艺参数的波动。 注塑参数波动影响性能。
长期老化。 材料性能会随时间下降。
实际使用的不确定性。 可能有超出设计的载荷。
所以验证时如果结果刚好达到要求，说明余量不足，应当调整设计而不是接受。
建议的做法是：设定一个内控的目标值，高于标准要求，验证时以内控值为准。
装配验证的重点
配合尺寸。 孔轴配合、卡扣配合的松紧。
装配顺序。 能否按设计的顺序装配。
工具可及性。 螺钉位置是否便于工具操作。
累积公差。 多个零件叠加后的尺寸。
拆卸的可行性。 维修时能否拆开。
「累积公差」在多零件装配中容易出问题。 单个零件都在公差内，装起来却干涉或者间隙过大。开模前用样件做一次完整装配，能发现这类问题。
与模具设计的配合
结构验证的结果应当反馈到模具设计：
加强的位置。 验证中发现的薄弱部位，在模具设计前加强。
壁厚的调整。 强度不足的部位增加壁厚或加筋。
脱模的考虑。 加强筋和结构调整要考虑脱模可行性。
浇口位置。 影响熔接线的位置，而熔接线是强度薄弱处。受力部位应当避开熔接线。
收缩与变形。 结构设计要考虑注塑收缩带来的变形。
「熔接线位置」是注塑件强度的实际影响因素，两股熔体汇合处的强度低于本体，如果这个位置恰好在受力部位，实际强度会低于预期。建议在模具设计阶段就与模具方讨论浇口位置与受力部位的关系。
开模后的验证
开模之后仍要验证，但重点不同：
首件验证。 尺寸、外观、装配。
样件的强度验证。 用实际注塑件重新验证强度，与开模前的预验证对比。
工艺窗口的确定。 注塑参数的适宜范围。
一致性验证。 不同型腔、不同批次的一致性。
「用实际注塑件重新验证」是必要的，因为注塑件的性能受工艺影响，与机加工件有差异。开模前的验证是为了降低风险，不能替代开模后的正式验证。
开模前验证的项目对照
| 项目 | 验证方式 | 不做的风险 |
|---|---|---|
| 关键件强度 | 机加工件加载 | 开模后强度不足 |
| 装配关系 | 快速成型件试装 | 干涉或间隙过大 |
| 卡扣功能 | 样件反复扣合 | 扣合力不当或断裂 |
| 活动机构 | 样件操作 | 卡滞或运动干涉 |
| 尺寸链 | 整套样件试装 | 累积公差超差 |
| 材料性能 | 材料试样测试 | 材料不达标 |
这张表可以作为开模前的检查清单，逐项确认是否已验证。
与供方的配合
开模前的验证往往需要模具方和材料供方配合：模具方对结构工艺性的意见；材料供方的性能数据与加工建议；以及试模阶段的参数调整。
建议在结构设计阶段就让模具方参与评审，他们对脱模、浇口、变形的经验能避免很多后期问题。
我们的做法
我们可以承接开模前的结构验证测试——用机加工件或快速成型件做针对性的加载试验，方法可以简化，目的是回答结构够不够强这个问题。
这类测试周期短、成本低，相比开模后发现结构问题的代价，是很划算的投入。
对于关键受力件，我们建议做加载至失效的试验并记录失效位置，这个信息对结构优化的价值大于单纯的合格判定。
如果你有结构件需要在开模前做验证，可以把图纸和受力情况发过来一起讨论方案，或者直接联系：132 4819 8029。检测能力见服务介绍，产品分类见手动轮椅检测与电动轮椅检测，案例见案例。
English version
Conclusion. Plastic and die-cast parts require tooling, and tooling costs both money and time. Once cut, scope for structural change is limited: minor dimensional adjustments can be made by modifying the tool, while structural changes generally mean cutting new tooling. What should be verified before cutting tooling is therefore whatever would require a structural change if it failed. The common error is completing the structural design, cutting the tool, and discovering on the first samples that strength is inadequate or assembly is problematic, by which point the cost of change is already high.
What to verify before cutting tooling. Strength and stiffness, calculated and verified, establishing whether key load-bearing parts carry the design load. Assembly relationships, covering fit with adjacent parts and feasibility of assembly. Function, covering movement of moving parts, engagement of clips and achievement of sealing. Dimensional chains, establishing whether accumulated tolerance causes assembly problems. Material confirmation, establishing whether the chosen material's actual properties suffice. And a preliminary judgement of manufacturability, covering wall thickness, draft and gate position. Of these, strength and assembly most need physical verification: calculation informs but does not fully substitute.
Methods without tooling. Several routes allow physical verification before tooling exists. Additive manufactured samples suit verification of assembly and dimensions, though their material properties differ greatly from moulded parts and they are unsuitable for strength verification. Machined parts, cut from sheet or bar of the same material, have properties close to reality and suit strength verification, though fibre orientation and internal stress differ from moulded parts. Soft tooling or simple steel tooling costs less than production tooling and produces parts close to the real thing. Similar existing parts allow verification of principle. And local specimens replicate only the critical loaded area for targeted strength verification. Machined parts for strength verification offer good value: despite differing from moulded parts, they answer the central question of whether this structure, in this material, at this wall thickness, is strong enough.
Focus of strength verification. Key load-bearing areas on the load path. Stress concentrations at holes, notches, abrupt thickness changes and rib roots. Assembly connections around screw bosses, clips and inserts. Thin sections reduced for draft or weight. And long cantilevers such as footplates and armrests. Verify by loading to failure, observing where and at what load failure occurs and judging the margin from that. Loading to failure is more valuable than a pass or fail at this stage, because it shows the size of the margin and the location of the weak point.
Judging the margin. Results before tooling should show more margin than at type testing, for several reasons. Samples differ from production parts, since process affects properties. Material batches vary in production. Process parameters fluctuate, affecting moulded properties. Materials age, with properties declining over time. And real use is uncertain, with loads possibly exceeding the design. A verification result that only just meets the requirement therefore indicates insufficient margin, and the design should be adjusted rather than accepted. Set an internal target above the standard requirement and verify against that.
Focus of assembly verification. Fit dimensions, covering the tightness of hole-and-shaft and clip engagements. Assembly sequence, establishing whether the designed order works. Tool access, establishing whether screw positions admit tools. Accumulated tolerance across stacked parts. And feasibility of disassembly for repair. Accumulated tolerance causes frequent trouble in multi-part assemblies: every part lies within tolerance while the assembly interferes or gaps excessively. A complete trial assembly with samples before tooling reveals such problems.
Feeding into tool design. Verification results should inform tool design. Reinforce where verification found weakness, before the tool is designed. Adjust wall thickness or add ribs where strength is inadequate. Consider draft when adding ribs and adjusting structure. Consider gate position, which determines weld line location, weld lines being weaker than the bulk material; loaded areas should avoid them. And consider shrinkage and distortion in the structural design. Weld line position genuinely affects moulded part strength: where two flow fronts meet the material is weaker, and if that coincides with a loaded area the actual strength falls below expectation. Discuss gate position against loaded areas with the toolmaker during tool design.
Verification after tooling. Verification continues after tooling with a different focus. First article verification covers dimensions, appearance and assembly. Strength verification on actual moulded parts repeats the assessment and compares against the pre-tooling work. Process window determination establishes the appropriate moulding parameter range. And consistency verification covers differences between cavities and batches. Re-verifying with actual moulded parts is necessary, because moulded properties depend on process and differ from machined parts. Pre-tooling verification reduces risk without substituting for formal verification afterwards.
How we handle it. We can perform structural verification before tooling, running targeted loading tests on machined or additive manufactured parts with simplified methods, aimed at answering whether the structure is strong enough. Such testing is short and inexpensive and represents good value against the cost of discovering a structural problem after tooling. For critical load-bearing parts we suggest loading to failure and recording the failure location, information more valuable for structural optimisation than a simple verdict.
Send us the drawings and loading conditions and we will work out an approach. Phone or WeChat: +86 132 4819 8029.